We have used calcium-sensitive fluorescent dyes to measure changes in intra cellular calcium concentrations in the intact brain. Changes in intracellular calcium determined with this approach can be correlated with changes in extracellular sodium levels determined simultaneously with ion-sensitive electrodes. Preliminary results show that in hypoxia changes in extracellular sodium levels occur well before changes in intracellular calcium are observed. Studies with perfused liver show that the dyes used for intracellular calcium determination leak out of the liver. The fluorescence can therefore not be used in the liver.